The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube having an internal magnetic shield with its shape modified to greatly reduce beam landing errors caused by an external magnetic field.
In general, a color cathode ray tube comprises an evacuated envelope (a glass bulb) formed of a panel portion having a faceplate, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed on an inner surface of the faceplate which includes a multiplicity of phosphor elements of three colors, a shadow mask having a multiplicity of apertures therein and spaced from the phosphor screen in the panel portion, a three-beam in-line type electron gun housed in the neck portion for generating three electron beams and projecting the electron beams through the shadow mask to the phosphor screen, a magnetic shield of generally truncated pyramidal, shape extending from the interior of the funnel portion into the panel portion and having openings on the shadow mask side thereof and the electron un side thereof, and a deflection device mounted in vicinity of a transition region between the funnel portion and the neck portion.
In operation of this type of a color cathode ray tube, when the electron beams emitted from the electron gun are subjected to an ambient magnetic field, particularly the earth""s magnetic field in the path to the shadow mask, the electron beam slightly deviates from its intended trajectory depending upon the magnitude and direction of the earth""s magnetic field and lands away from its intended landing position on the phosphor screen, resulting in a landing error.
FIG. 5 illustrates the mechanism of occurrence of such a landing error of the electron beam. As shown in FIG. 5, the electron beam 51 emitted from the electron gun (not shown) is bent in the intended direction by the deflecting action on the beam by the deflection yoke (not shown) from the center of deflection 0 in the intended direction, goes straight in that direction, passes through one of the beam apertures, 52A, in the shadow mask 52 and impinges upon the intended landing position P on the phosphor screen 53 formed on the inner surface 6f the faceplate 1A.
When the electron beam 51 is not subjected to any influence of external magnetic fields, the electron beam 1 travels the normal trajectory as indicated by a solid line in FIG. 5. But if the electron beam 51 is subjected to the influence of an external magnetic field, the electron beam 51 deviates slightly from its normal trajectory, travels a trajectory different from the normal trajectory as indicated by a broken line in FIG. 5 and consequently lands on the position Pxe2x80x2 displaced a distance xcex94e from the intended landing position P on the phosphor screen 53, resulting in occurrence of a so-called beam landing error. The beam landing error deteriorates white uniformity and bright uniformity severely as well as color purity of a displayed image.
In practice, it is known that occurrence of the beam landing error in a color cathode ray tube can be suppressed by shielding the electron beam trajectories in the color cathode ray tube from the influence of external magnetic fields, specifically by employing a magnetic shield extending from the interior of the funnel portion into the panel portion. Now the magnetic shield is indispensable for color cathode ray tubes.
FIGS. 6A to 6C respectively are perspective views of different examples of magnetic shields which have been used for prior art cathode ray tubes. As shown in FIGS. 6A to 6C, magnetic shields 60 used for prior art cathode ray tubes are a magnetic shield of generally truncated pyramidal shape and have openings on the shadow mask side thereof and the electron gun side thereof.
While all their openings on the shadow mask side are rectangular with its entire edges of both the long and short sides contained in a single plane, the openings 61 on the electron gun side differ from shield to shield in FIGS. 6A to 6C. The four sides of the generally rectangular opening 61 on the electron gun side are level in the magnetic shield 60 of FIG. 6A, those in FIG. 6B are provided with V-notches, and those in FIG. 6C are provided with U-notches.
Even when an ambient magnetic field, especially the earth""s magnetic field is present in a place where the color cathode ray tube is used, such a magnetic shield 60 produces a magnetic flux only through the material of the magnetic shield 60 made of a magnetic metal, and the interior of the magnetic shield 60 is shielded from the external (earth""s) magnetic field and the electron beam traveling within the magnetic shield 60 is immune against the influence of the external (earth""s) magnetic field. High-permeability materials are generally used for the magnetic shield 60 to effectively concentrate the external (earth""s) magnetic field.
Recently, the demand for higher definition phosphor screens is becoming greater with the development of manufacturing technology of color cathode ray tubes, and the tolerance for beam landing on the phosphor elements is becoming considerably smaller with an increasing degree of the high definition. Consequently consideration has been required to a magnetic shield in the recent color cathode ray tubes to improve immunity against changing magnetic field environments.
As described above, magnetic shields 60 used for prior art cathode ray tubes have been of the generally truncated pyramidal shape. The four sides of their generally rectangular opening 61 on the electron gun side thereof have been level as shown in FIG. 6A, have been provided with V-notches as shown in FIG. 6B, or have been provided with U-notches as shown in FIG. 6C.
It can be thought that a magnetic circuit is formed by the magnetic shield 60 and the magnetic core of the deflection yoke mounted in the vicinity of the magnetic shield 60.
In this specification, a distance between an end of a magnetic core of the deflection device facing toward the faceplate and an end of the magnetic shield facing toward the plural-beam in-line type electron gun, which is measured in a section plane containing a longitudinal axis of the cathode ray tube and being inclined by an angle xcex8 with respect to a horizontal scanning direction of the electron beams, is hereafter referred to merely as a magnetic shield-core distance D, and the angle xcex8 is hereafter referred to as a section plane angle xcex8.
The magnetic shield-core distance D is not approximately constant with variation of the section plane angle xcex8 in magnetic shields 60 used for the prior art color cathode ray tubes. In FIG. 3, the rectangular curve and the circular curve indicate the relationship between the magnetic shield-core distance D and the section plane angle xcex8 in the polar-coordinate form in color cathode ray tubes for a prior art shield 60 and an embodiment of a magnetic shield 7 of the present invention to be explained later, respectively. In FIG. 3, reference numeral 8B denotes the magnetic core of the deflection yoke.
In FIG. 3, the magnetic shield-core distances D measured at the center MH of the long (horizontal) side, at the center MV of the short (vertical) side, and at the corner of the generally rectangular opening on the electron gun side are represented by DHM, DVM, DC, respectively, in the prior art color cathode ray tube. The following inequality is satisfied:
DHM less than DVM less than DC 
The magnetic shield-core distance D progressively increases from DHM toward DC as one goes from the center MH toward the corner and the magnetic shield-core distance D progressively increases from DVM toward DC as one goes from the center MV toward the corner.
When the magnetic shield-core distance D is not constant with variation of the section plane angle xcex8 in magnetic shields 60 used for the prior art color cathode ray tubes, the magnetic resistance varies with the distance D, that is, with the section plane angle xcex8. The magnetic flux of the external magnetic field is concentrated at portions of low magnetic resistance, does not pass uniformly through the entire magnetic shield 60, the magnetic shield 60 is not expected to provide a sufficient shielding effect, the electron beam 51 is subjected to a slight influence of the external (earth""s) magnetic field within the magnetic shield 60 and deviates slightly from its intended trajectory.
The prior art color cathode ray tube cannot be completely shielded from the external (earth""s) magnetic field by its magnetic shield 60, the electron beam 51 (see FIG. 5) is subjected to the influence of the external magnetic field, deviates slightly from its intended trajectory, and moves slightly away from its intended landing position P to a position Pxe2x80x2 on the phosphor screen 53, resulting in occurrence of a landing error. In the prior art color cathode ray tube, the landing error causes a problem in that color purity, and consequently white uniformity or brightness uniformity of the displayed image deteriorate.
An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a color cathode ray tube capable of suppressing occurrence of beam landing errors sufficiently by increasing the shielding effect against the external magnetic fields by the magnetic shield greatly.
To accomplish the above object, the magnetic shield of generally truncated pyramidal shape in a color cathode ray tube of the present invention is configured such that a magnetic shield-core distance between an end of a magnetic core of the deflection device facing toward the phosphor screen and an end of the magnetic shield facing toward the electron gun is approximately constant for section plane angles xcex8 in a range of 0xc2x0 to 360xc2x0, the magnetic shield-core distance being measured in a section plane containing a longitudinal axis of the cathode ray tube and being inclined at the section plane angle xcex8 with respect to a horizontal scanning direction of the electron beams.
Further, to accomplish the above object, the magnetic shield of generally truncated pyramidal shape in a color cathode ray tube of the present invention can be configured so as to satisfy a following inequality,
0.75xc3x97Davexe2x89xa6(Dmax+Dmin)/2xe2x89xa61.25xc3x97Dave, 
where Dmax, Dmin, and Dave are respectively a maximum, a minimum, and an average of magnetic shield-core distances between an end of a magnetic core of the deflection device facing toward the faceplate and an end of the magnetic shield facing toward the plural-beam in-line type electron gun, the magnetic shield-core distances being measured in a section plane containing a longitudinal axis of the cathode ray tube and being inclined at the section plane angle xcex8 with respect to a horizontal scanning direction of the electron beams.
For the purpose of simplifying the design of the magnetic shield, Dave can be set to be (Dverm+Dhorm+Dcor)/3, where Dverm and Dhorm are the magnetic shield-core distances measured at the section plane angles 0xc2x0, 180xc2x0 (horizontal section plane) and 90xc2x0, 270xc2x0 (vertical section plane), respectively, and Dcor are the magnetic shield-core distance measured in the section plane intersecting a corner of the generally rectangular end of the magnetic shield facing toward the electron gun.
Further, to accomplish the above object, the magnetic shield of generally truncated pyramidal shape in a color cathode ray tube of the present invention can be configured so as to satisfy following inequalities,
0.5xc3x97Davexe2x89xa6Dmin, and Dmaxxe2x89xa6Davexc3x971.5, 
where Dmax, Dmin, and Dave are respectively a maximum, a minimum, and an average of magnetic shield-core distances between an end of a magnetic core of the deflection device facing toward the faceplate and an end of the magnetic shield facing toward the plural-beam in-line type electron gun, the magnetic shield-core distances being measured in a section plane containing a longitudinal axis of the cathode ray tube and being inclined at the section plane angle xcex8 with respect to a horizontal scanning direction of the electron beams.
For the purpose of simplifying the design of the magnetic shield, Dave can be set to be (Dverm+Dhorm+Dcor)/3, where Dverm and Dhorm are the magnetic shield-core distances measured at the section plane angles 0xc2x0, 180xc2x0 (horizontal section plane) and 90xc2x0, 270xc2x0 (vertical section plane), respectively, and Dcor are the magnetic shield-core distance measured in the section plane intersecting a corner of the generally rectangular end of the magnetic shield facing toward the electron gun.
According to the present invention, a magnetic resistance of a magnetic circuit formed by a magnetic shield and a magnetic core of the deflection yoke mounted in the vicinity of the magnetic shield is approximately constant, or is held to be varying within a range in which the influence of the external magnetic field is substantially suppressed, along the entire circumference of the faceplate side end of the magnetic core, the flux produced by the external field pass approximately uniformly through the entire magnetic shield and the magnetic shield provides a sufficient shielding effect against the external magnetic field.
As a result the electron beam is immune against the influence of the external magnetic field and occurrence of beam landing errors can be suppressed sufficiently.